The material aluminum borosilicate (“abs”), includes selected fractions of alumina (Al2O3), boria (B2O3) and silica (SiO2), and was studied experimentally by H. Sowman in or around 1970 and disclosed in U.S. Pat. No. 3,795,524 (the “Sowman patent”). The Sowman patent focuses on a particular trapezoid (ABCD) in the phase diagram for Al2O3, B2O3 and SiO2, illustrated in the Sowman patent, FIG. 1, which extends from a boundary line segment connecting the vertices Al2O3 and B2O3 to a second boundary line adjacent to the SiO2 vertex (approximately 65 percent weight fraction SiO2 and the remainder a mixture of Al2O3 and B2O3. Continuous fibers of abs, up to 600 cm in length, with diameters of about 15 μm, can be produced by a variation of the Sowman patent process at temperatures of about 900° C. (1652° F.) Amorphous abs is produced at temperatures as low as 600° C. (1040° F.) and is more likely to form near the Al2O3—B2O3 and boundary line segment in the abs phase diagram.
Largely crystalline abs (e.g., 9Al2O3+2B2O3, confirmed by X-ray diffraction) is produced at temperatures of 1200° C. (2120° F.) and above. A material composition that is predominantly 3Al2O3+B2O3 is preferred for some applications. At these higher temperatures and/or for abs compositions lying further from the boundary line segment Al2O3—B2O3 in the abs phase diagram, the resulting abs tends to be more porous, to be more fragile, to have a reduced elasticity modulus, to begin to lose its optical transparency, and to become more opaque. An abs compound can be formed as an aqueous solution or as a two-phase solution (mixture of colloidal dispersion and water-soluble alumina and boria). An aqueous solution can be used to produce abs fibers if the solution is first made denser and more viscous and converted to a gel. Fibers of abs can be woven into a fabric-like material.
For thermal protection of a re-entering space vehicle, where the leading edges may experience temperatures up to T=3200° F. (1760° C.) for time intervals as long as 600 sec, a material or additive is needed that will survive under these conditions and will maintain reasonably good radiation emittance to aid in partial cooling of the leading edges. The material should also permit reduction of any mismatch in thermal expansion coefficients (CTE) across an material interface located near a leading edge. Ideally, this material would permit the thermal protection configuration to be used more than once, with at most minor re-processing or material replacement.